window.onload =function(){
	let banner = document.getElementById('banner');
	banner.style.backgroundImage='url("img/a1.jpg")';
	banner.style.backgroundSize = '100% 100%';
	let index = 2;
	setInterval(()=>{
		banner.style.backgroundImage='url("img/a'+index+'.jpg")';//引入不用加../
		
		index++;
		if(index>6){
			index = 1;
		}
	},2000)
}